Tissue microarray (TMA) technology was first described by Wan and Furmanski (Wan et al., 1987), and later advanced by Kononen and Kallioniemi in 1997 with production of an apparatus for mass-production of TMAs (Kononen et al., 1998). Consisting of an ordered array of tissue cores—up to 2,000—on a single glass slide, tissue microarrays provide a mechanism for a maximal use of scarce tissue resources. Most tissue microarrays are currently constructed from pathology tissue block archives, and the coordinate clinical data can be correlated with experiments performed on these tissues. TMAs allow for the validation of new concepts in cell and molecular biology on human tissue (Rimm et al., 2001a; Rimm et al., 2001b) and have been considered the ultimate step in the association of gene expression information with human tissues and human disease.
With over 30,000 genes within the human genome, encoding over 100,000 proteins, the task of sorting the vast number of gene and protein targets to identify those with clinical relevance and diagnostic, prognostic, and/or therapeutic potential which are thereby promising pharmaceutical targets is overwhelming. Target validation is an important step and has traditionally been done with assays such as Northern blot analysis, RT-PCR, macroarray, microarray, and gene chips. These technologies simply provide evidence of differential expression of specific genes. For most techniques in molecular biology, tissue is homogenized to isolate RNA or protein for expression analysis. Unfortunately, the tissue obtained is not necessarily composed solely of the cells of interest. The tissue homogenate can contain normal cells, tumor cells, blood cells, muscle cells, and other cell types that may result in misleading information. Additionally important spatial information that is the context of expression within cells and tissues is lost when using these techniques.
Tissue microarrays supply a mechanism for conservation of tissue, while providing the ability to evaluate hundreds of archival tissue specimens on a single microscope slide. By exposing all tissues of a tissue microarray to precisely the same conditions, the slide-to-slide variability inherent to immunohistochemistry and in situ hybridization is minimized.
In an exemplary process, the target tissues are core-biopsied with a 0.6-1.5 mm diameter needle under the guidance of a pathologist. The cores are then arranged in a ‘recipient’ paraffin block. The maximum number of specimens one block can hold varies with core size. For example, up to 60 cores are possible with 1.5 mm needles and up to 2000 or more with new smaller diameter needles. The most common size is about 0.6 mm allowing a maximum of about 750 tissue core section, commonly referred to as histospots. The block containing the array is sectioned in an identical fashion to any paraffin-embedded tissue block. The maximum number of sections a block can provide depends on the size of the original tumor and the skill of the histotechnologist, but it is not uncommon to obtain hundreds of sections from a single conventional specimen (Rimm et al., 2001a; Rimm et al., 2001b).
Unlike traditional tissue analysis techniques, which use at least one slide for every tissue from each patient or test subject, TMA technology offers the benefits of: (1) conservation of precious tissue resources, (2) improved internal experimental control, (3) reduced consumption of reagents, and (4) facilitation of multi-center research studies.
TMA studies and those done using whole tissue sections (WTS) frequently employ sophisticated methods for scanning the processed tissues as well as archiving and analyzing the resulting visual data. However, the majority of scientists still analyze tissue specimens in a traditional fashion using microscopes. Manual inspection on a microscope to interpret staining results involves multiple manual steps, lacks standardization, and is slow. A few devices have been developed in recent years to allow for quantitative, efficient, and specific analysis of data generated from TMAs and WTS. However quality assessment of each tissue specimen is still done by a pathologist or trained scientist to confirm for example that tissue samples are present and not artifactually damaged and that staining is even and reproducible across the slide. Typically after staining, a specimen is reviewed by a pathologist or technician viewing each individual histospot or a whole tissue sample across multiple fields of view (FOV) in a traditional fashion using microscopes. The pathologist or technician provides a subjective assessment of the quality of the particular spot, FOV or WTS and whether it should be included or excluded from further analysis. For example, manual validation of a multi-spot TMA that contains for example 700 or more spots can easily take eight hours of a professional's time. Even amongst trained professionals, manual validation is not consistent due to the subjective nature of the evaluation. Better methods for assessing quality of histological tissue sections, including TMA specimens prior to analytical analysis are clearly needed.